1. Field of the Invention
The present invention relates generally to carbon-carbon composites and more particularly to the protection of carbon composites from oxidation and degradation at high temperatures.
2. Description of the Background Art
Carbon-carbon composites are strong, lightweight, high temperature materials that are used as ablators in short duration rocket and reentry systems and are currently being developed for structural applications in advanced missiles, aircraft, and aerospace vehicles. Carbon-based materials are desirable because of the high strength and low weight density of carbon. Furthermore, carbon maintains its mechanical properties at high temperatures up to 2000.degree. C. Many future applications for C--C composites require operation at elevated temperatures in an oxidizing environment. Carbon erodes rapidly in air at temperatures as low as 400.degree. C. Much effort is thus being devoted at developing techniques for protecting C--C composites against oxidation at elevated temperatures. An effective method of protecting carbon from oxidation is to establish a barrier against oxygen penetration in the form of an external coating. The development of external coatings such as ceramics to protect C--C composites was initiated about 20 years ago to provide reusable thermal protection for the shuttle orbiter vehicles. Because of the large differences in thermal expansion characteristics of carbon fibers and ceramic materials, few coatings have been found to withstand thermal cycling without cracking. The strategy that has proven most successful dealing with cracked external coatings is to employ a boron-rich inner coating beneath the cracked outer coating that acts as the oxygen barrier. In this scheme, oxygen penetrating the crack oxidizes the boron layer to produce a compliant sealant glass (B.sub.2 O.sub.3) that fills and seals the crack. Prominent coating combinations consist of a SiC outer coating and boron-rich inner coatings that consist of elemental boron or B.sub.4 C. The silicon-based ceramics are used for outer coatings because of their excellent oxidative stability, refractoriness, and relatively low thermal dimensional changes.
Another approach often used in combination with external barrier coating is to add elemental boron, B.sub.4 C, SiC, and phosphorous compounds to precursor carbon matrix material during processing. On exposure to air at elevated temperatures, these additives are expected to oxidize and provide in-depth oxidation protection. Experience has shown that it is difficult to achieve a uniform dispersion of the particulate additive throughout the composite and substantial amounts of the carbonaceous material are oxidized before the additive can become effective.